JIRA Import Wizard - purpose of Map Field Value Check Box

I have gotten the import wizard working nicely and pretty much understand how to use it. But, I do not understand why someone would want to select the Map Field Value check box. That brings up antohe screen whcih lets you enter other values. For my work I am mapping the fields in the csv to Jira, leaving the Map Field Values unchecked and using everything as is from the CSV. Why would one use Map Field Value Check box?

As a Belgian, beer-lover and home brewer, beer is one of my great passions. I love the fact that with just a few ingredients (usually just water, hop and malt) you can create so many different tastes...